1. Field of the Invention
This invention relates in general to swivel equipment for transferring fluids. In particular, the invention relates to a fluid swivel joint for a swivel stack assembly adapted for transferring fluids between tankers, storage vessels and the like and one or more conduits beneath the ocean surface. The fluid of the swivel may be product such as hydrocarbons to be transferred from the seabed to a vessel or may be water or gas to be transferred from the vessel to the seabed for well stimulation.
Still more particularly, the invention relates to a sealing arrangement for a fluid swivel joint which uses the mechanical design of the joint with the pressure of the fluid flowing through the joint to substantially prevent seal glands, and extrusion gaps in which dynamic seals are placed, from enlarging as a function of high pressure of the fluid commonly encountered on offshore loading terminals for oil and gas tankers.
2. Description of the Prior Art
The offshore search for oil and gas has greatly expanded in recent years and progressed into deep rough waters such as the North Sea. To facilitate production of oil and gas from remotely located offshore fields, complex mooring systems for offshore loading terminals, which serve as centralized production sites for the entire field, have been developed. Flexible fluid lines, called risers, extend from a subsea location to the mooring site to permit the transfer of fluids between a moored vessel and a subsea location. For example, certain fluid lines may be used to convey oil and gas into the floating vessel while other fluid lines may be used to inject liquids or gases back from the vessel into subsea wells for purpose of control, well stimulation, or storage.
Floating vessels can be moored to a single point mooring system, which permits the vessel to weathervane and rotate 360° about a single mooring point. To permit the vessel to rotate and move freely without causing twisting or entanglement of the various risers to which the vessel is attached, it is necessary to provide a swivel mechanism to connect the fluid lines to the mooring site. Furthermore, since a plurality of risers are involved, it is necessary that swivels be stacked in order to have the capability of accommodating multiple fluid lines or risers.
Separate swivel assemblies are stacked on top of each other with a swivel base fixed to a stationary frame anchored to the sea floor.
Prior high pressure fluid swivels have provided an inner housing and an outer housing which is rotatively supported on the inner housing by a bearing so that the outer housing is free to rotate about the inner housing. An annular conduit chamber or passage is formed between the two housings when the two housings are placed in registration with each other. An inlet from the inner housing communicates with the chamber, and an outlet in the outer housing communicates with the chamber. Upper and lower dynamic seals in the form of face seals or radial seals are placed in grooves or gaps between co-axially opposed or radially opposed surfaces of the inner and outer housings to prevent fluid from leaking past the two facing surfaces while the high pressure fluid is present in the chamber.
When high pressure is present in the inlet and passes through the annular passage and out the outlet, the pressure in the annular passage acts to separate the inner housing and the outer housing from each other. In other words, the inner housing is forced to contract radially inward as a consequence of the force generated by the fluid pressure acting on an effective area between the two dynamic seals; the outer housing is forced to expand radially outward by the force of the fluid pressure acting on an effective area between the upper and lower dynamic seals. Separation occurs between the facing surfaces as a result of high fluid pressure in the annular passage. High pressure as used herein is meant to be at the level of 2,000 psi and above.
As the pressure of flowing fluid increases, the separation between the facing surfaces in which the seals are placed increases. Such separation can be large enough, due to the high fluid pressures, so as to prevent leak-free operation of the swivel at the high pressures by seal extrusion failure.
Swivel component deformation has been the subject of much effort by prior developers. The prior art has considered the idea of adding more material to the swivel components so that deformation as a function of pressure—especially high pressure in the 5,000 to 10,000 psi range—will resist deflection. With high pressures, however, the swivel components, i.e., the inner and outer housings, become so large and heavy that they are disadvantageous from weight, cost, handling, and size standpoints, and without necessarily achieving the desired extrusion gap control.
The prior art discloses swivels that use exterior pressure sources to apply balancing or “barrier” fluid pressure at the dynamic seal interface. Examples of such “active” pressure compensation for dynamic seal gap control are shown in U.S. Pat. No. 4,602,806 to Saliger; U.S. Pat. No. 4,669,758 to Feller et al., U.S. Pat. No. 5,411,298 to Pollack; U.S. Pat. No. 6,053,787 to Erstad et al., and U.S. Pat. No. 4,662,657 to Harvey et al. All of these patents disclose separate anti-extrusion rings above and below the annular fluid passage in combination with active pressure compensation.
U.S. Pat. No. 4,555,118 to Salinger discloses, at FIG. 4, a free floating anti-extrusion ring placed above and below an annular passage between inner and outer rings. The free floating anti-extrusion ring is initially displaced (i.e., at zero pressure) from the inner joint ring by a small seal extrusion gap. In operation, the internal pressure of the pressurized fluid in the annular passage is transmitted to the outer side of the anti-extrusion ring such that the pressure differential across the seal presses the anti-extrusion ring against the outer surface of the inner ring. In other words, the seal extrusion gap width varies as a function of internal pressure. Metal to metal contact of the anti-extrusion ring with the annular surface of the inner ring can cause friction and scoring problems during operation.
U.S. Pat. No. 4,819,966 to Gibb, at FIGS. 2, 3, and 4, shows an annular ring having an annular groove which registers with the inlet of an inner housing. An annular chamber is formed outwardly in the annular ring such that upper and lower lips are created in the annular ring that face the exterior surface of the inner housing. The lips carry dynamic seals and are forced into sealing engagement about the cylindrical surface of the inner housing above and below the inlet when pressure is in the chamber. A constant radial seal gap is maintained as a function of pressure by proper shaping of the chamber and the ring and the lip. A lubricating system may also be provided for injecting a controlled fluid.
U.S. Pat. No. 6,450,546 to Montgomery and Roy shows a sealed fluid joint for a fluid swivel in which a pressure balanced middle housing ring is mounted between an inner housing and outer housing ring. Pressure balance is achieved by providing an inner annulus chamber or cavity between the inner housing and middle housing ring and an outer annulus chamber or cavity between the middle and outer housing ring. Holes or passages through the middle housing ring fluidly connect the inner and outer chambers. Dynamic seals are placed in seal glands between the inner housing and middle housing ring. Static seals are placed in seal glands between the middle and outer housing rings. The arrangement transfers component deformation due to product fluid pressure from the dynamic seal interface to the static seal interface by exposing fluid product pressure to a smaller effective area at the dynamic seals on the inner side of the middle housing ring than an effective area at the static seals on the outer side of the middle housing ring. The counter forces generated by the product fluid pressure over two different effective areas on the middle housing ring deforms the middle housing ring radially in a predetermined direction and amount as a function of increasing pressure. Control of radial deformation of the middle housing ring is passive, because it depends on a geometrical arrangement of dynamic and static seals on both sides of the middle housing ring and is proportional to the product fluid pressure.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a fluid swivel arrangement that is capable of flowing high pressure product through it while minimizing product leaking past dynamic seal grooves formed between inner and outer housings.
Another object of the invention is to provide a fluid swivel arrangement for a predetermined high pressure rating, path diameter, and arrangement and shape of the components that minimizes the swivel outside diameter, height, and weight.
Another object of the invention is to provide shapes and arrangements of inner and outer housings with seals between them that minimize relative internal deflections so as to assume proper function and long life of the seals.
Another object of the invention is to provide a fluid swivel arrangement with inner and outer housings arranged so that internal areas subjected to high fluid pressure are minimized, thereby allowing the parts to be reduced in size and weight.
Another object of the invention is to provide dynamic upper and lower seal recesses in the outer housing with face seals placed therein where the seal recesses are arranged concentrically outward from the axial center line of the fluid swivel, with the shape of an annular passage in the outer housing compensating for the tendency of clearances behind the seals to open wider axially as high pressure acts inside the swivel.
Another object of the invention is to match the bending stiffness of the inner housing with the bending stiffness of the outer housing such that elastic matching occurs with the inner and outer housings axially deflecting about the same amount in the same direction when high pressure fluid is carried by the swivel, so that the inner and outer housings effectively move together with increasing pressure and the axial clearance between the parts remains almost the same.
Another object of the invention is to provide upper and lower dynamic seals where the upper dynamic seals are characterized by a slightly larger diameter than the opposing lower dynamic seals, thereby providing a positive downward force to prevent the outer housing from floating upwardly, thereby preventing excessive vertical force of the seals against the surfaces of the outer housing.